1. Field of the Invention
The invention generally relates to the field of ultrasonic generators and, more particularly, to a system and method for processing heterodyned ultrasonic signals that are generated during valve inspections.
2. Description of the Related Art
It is well known that ultrasonic generators and detectors can be used to locate leaks or defects, e.g., in pipes. Such a system is shown in U.S. Pat. No. 3,978,915 to Harris. In that arrangement, ultrasonic generators are positioned in a chamber through which the pipes pass. At the ends of these pipes, exterior to the chamber, ultrasonic detectors are located. At the point where a leak occurs in the pipe or the pipe wall is thin, the ultrasonic energy will enter the pipe from the chamber and travel to the end of the pipe where the detector is located. The detector will receive an ultrasonic signal at the end of the pipe indicating the existence of the leak or weak spot in the pipe.
By locating an ultrasonic generator in a closed chamber, a standing wave pattern with peaks and nodes is established. If a node occurs at the position of a leak or weak spot, no ultrasonic energy will escape and the defect will not be detected.
Ultrasonic sensors have also been used to detect ultrasonic energy generated by friction within mechanical devices as disclosed in U.S. Pat. No. Re. 33,977 to Goodman, et al., the details of which are hereby incorporated herein, in their entirety, by reference. The greater the amount of friction, the greater the intensity of the generated ultrasonic energy. Applying a lubricant to the device reduces friction and consequently the intensity of the generated ultrasound drops. Measuring ultrasonic energy thus provides a way to determine when lubrication has reached the friction generating surfaces. Additionally, faulty devices, such as bearings, generate a higher level of ultrasonic energy than do good bearings and thus, this condition can also be detected. However, conventional means require two people to perform this procedurexe2x80x94one person to apply the lubricant to the device, and one person to operate the ultrasonic detector.
In certain instances, e.g., when detecting the malfunction of bearings, an ultrasonic detector is mechanically coupled to the casing of the bearings so that the vibrations caused by the malfunction can be mechanically transmitted to it. With such an arrangement, the frequency is not set by an ultrasonic generator, but is created by the mechanical vibration itself. Here, an ultrasonic detector circuit must be capable of sweeping over a band of frequencies to locate the one frequency that is characteristic of the malfunction. This is usually accomplished by a heterodyning circuit which can be tuned to various frequencies, much in the manner of a radio receiver.
Since ultrasonic energy used for these purposes is generally in the range of 40 kHz, it is too high in frequency to be heard by a human being. Thus, means are typically provided for heterodyning, or frequency shifting, the detected signal into the audio range, and various schemes are available for doing this.
Ultrasonic transducers generally produce a low voltage output in response to received ultrasonic energy. Thus, it is necessary to amplify the detected signal using a high-gain preamplifier before it can be accurately processed. However, if low cost heterodyning and display circuitry are to be used, means must be made available to attenuate the amplified signal to prevent saturating these circuits when high input signals are present. This attenuation also adjusts the sensitivity of the device. For a hand-held unit, the degree of attenuation should be selectable by the user. For example, U.S. Pat. No. 4,785,659 to Rose et al. discloses an ultrasonic leak detector with a variable resistor attenuator used to adjust the output level of an LED bar graph display. However, this attenuation method does not provide a way to establish fixed reference points to allow for repeatable measurements.
U.S. Pat. No. 5,089,997 to Pecukonis discloses an ultrasonic energy detector with an attenuation network positioned after an initial pre-amplifier and before the signal processing circuitry, which creates an audible output and an LED bar graph display. The resistors in the Pecukonis attenuation network are designed to provide an exponential relationship between the different levels of attenuation. However, Pecukonis does not heterodyne the detected signals to produce an audible output, but rather teaches the benefits of a more complex set of circuits which compress a broad range of ultrasonic frequencies into a narrower audible range. For many applications, the cost and complexity of this type of circuitry are not necessary.
When using ultrasonic energy to detect leaks, it is useful to have a portable ultrasonic sensor which indicates the presence and intensity of ultrasonic energy both visually and audibly. U.S. Pat. No. Re. 33,977 to Goodman et al. discloses an ultrasonic sensor that displays the intensity of the detected signal on an output meter operable in either linear or logarithmic mode, and also provides for audio output through headphones. U.S. Pat. No. 4,987,769 to Peacock et al. discloses an ultrasonic detector that displays the amplitude of the detected ultrasonic signal on a ten-stage logarithmic LED display. However, the detector disclosed in Peacock does not process the detected signal to produce an audible response, nor does it provide for signal attenuation after the initial pre-amplification stage.
Means have been proposed for increasing the output of the ultrasonic transducer. For example, in U.S. Pat. No. 3,374,663 to Morris it is suggested that an increase in the voltage output can be achieved by serially arranging two transducers. It has been found, however, that with such an arrangement a typical transistor pre-amplifier loads the transducers to such an extent that the gains achieved by stacking them serially are lost. The Morris patent proposes the use of a triple Darlington configuration in order to produce a sufficiently high input impedance to prevent this degradation in the signal produced by the stack of transducers. Unfortunately, the transducers in this arrangement are not placed so that they both readily receive ultrasonic energy. Thus, the Morris arrangement is not entirely satisfactory.
The present invention is directed to providing improved methods and apparatus for detecting leaks and mechanical faults by ultrasonic means. In accordance with the invention, an input transducer signal is applied to a unity gain buffer amplifier that is used to maintain the impedance level seen by the transducer. The processed signal from the unity gain buffer amplifier is supplied to a voltage control amplifier that also receives a voltage control signal that is generated by a digital-to-analog converter located on an external I/O board. The voltage control signal is used to switch the voltage controlled amplifier such that the dynamic range of the signal is expanded prior to a clip of the signal. The voltage control signal is based on a level that is programmed into the voltage control amplifier by the digital-to-analog converter located on the external I/O board. The voltage controller is thus controlled by the I/O board in response to commands sent to the external I/O board from a micro-controller.
The output from the voltage controlled amplifier is connected to a fixed gain differential amplifier. The output signal from the fixed gain amplifier is supplied to a variable gain amplifier that is switchable between two fixed levels, such as 0 dB and 20 dB. The gain level of the variable gain amplifier is toggled between the two fixed gain levels based on a level that is determined by the amount of gain that is programmed into the voltage control amplifier.
The output of the variable gain amplifier is supplied to a pair of heterodyning circuits, i.e., a dual heterodyning circuit. At each respective heterodyning circuit, the output signal from the variable gain amplifier is multiplied with a local oscillator signal that is internal to each circuit. Here, each local oscillator is nominally set to 38 kHz such that for a 40 kHz input transducer signal, a difference frequency of about 2 kHz (i.e., the audio component) is provided at the output of each heterodyning circuit.
The output signal from the first heterodyning circuit is amplified and divided into two signal branches. The first signal branch is transformer coupled to a headphone output. The second signal branch is connected to an amplifier that is also transformer coupled to a line output and also applied to an external audio amplifier. The output from the second of the heterodyning circuits is amplified and supplied to a metering circuit.
In addition, a further analog signal path is created at the second heterodyning circuit. The signal in this path is converted to a linear dB format analog signal and supplied to a micro-controller. This analog signal is converted in the micro-controller into a digital signal by an analog-to-digital converter, and is further converted in the micro-controller into a WAV file format, as well as other digital signal formats, for subsequent spectral analysis.
The present inventors have determined that a heterodyned signal that drives a meter requires a relatively large dynamic range, but a limited frequency response, while a heterodyned signal that is required for headphones or spectral analysis may have a low dynamic range, but requires high resolution. Further, it has been found that the resolution or frequency response of the input transducer signal is degraded if a single heterodyning circuit is used to drive a number of circuits or meters with competing requirements. In order to overcome these competing requirements, the present invention uses a dual heterodyning circuit in which the two individual heterodyne circuits are separately optimized so that the second results in a signal with a large dynamic range and the first results in a signal with a great resolution, and neither unduly loads the transducer array or obscures subtle frequency components. This permits the capture of particularly low level frequency components for extraction during spectral analysis.
In accordance with the invention, the first heterodyning circuit has a feed back loop filter and a transformer to provide an enhanced spectral (i.e., frequency) response. This circuit is used to drive the headphone, a wave file generator and a line output. This signal, which has a modest dynamic range but a high frequency response and a low signal to noise ratio, allows the spectrum of the signal to be analyzed in real time by an external spectrum analyzer, recorded for later analysis or listened to in real time through the headphones.
The second heterodyning circuit has a smaller frequency response but a larger dynamic range so that it can drive the meter. In accordance with the invention, the second heterodyne circuit is not required to have an optimized spectral response. If the meter were driven with the first heterodyne circuit, the impedance and dynamic range requirements of the meter would adversely affect the response. Thus, two heterodyne circuits are used, with the circuit that drives the meter being simpler, and less costly to manufacture and having a larger dynamic range.
In either mechanical analysis or electrical equipment analysis, a large number of frequencies in the low frequency range become lost. This is especially true in the case of electrical applications. After extended use of the detection equipment, operators often tend to begin to use their ears as a guide to the condition of an area of concern. However, it is extremely difficult for a person to discern with their ears the differences between inputs that are electrical in nature and inputs that are vibrational. Further, in other technologies, such as vibration analysis, infrared technologies, or where rotational equipment is used, the use of the human ear is a highly unreliable way in which to predict faults. For example, a transformer resonating at 60 Hz may cause a component in an equipment cabinet to resonate at the same 60 Hz. When an operator listens to the cabinet containing the component that is vibrating at the 60 Hz, it is impossible to determine whether the resonance is electrical or mechanical.
Typically, on/off valves are checked for their position, e.g., open or closed, or for leakage in the closed position. By way of a contact module, such valves can be tested using the portable ultrasonic device of the present invention. Contact modules are generally used to detect structure borne ultrasound. Transducers are contained in the module, and a rod is attached to the ultrasonic device. The rod acts as a waveguide that conducts ultrasound to excite the transducers to generate a signal that the ultrasonic device can measure.
A high level of confusion occurs when checking valves for leaks or xe2x80x9cbypassingxe2x80x9d caused by turbulent flows, either upstream or downstream from the valve in question. When measurements are performed at a distance from the valve, confusion can arise because an operator is unable to determine whether the valve is good or bad. This occurs because the operator is unable to distinguish between problems that occur at the remote location and those which occur at the valve itself. In addition, if the user only checks the upstream and downstream sides of a valve, then confusion may arise because they may believe the valve is xe2x80x9cbypassingxe2x80x9d and thus, change the valve. A valve is bypassing if fluid (gas or liquid) passes through the valve when it is closed.
If the operator falsely concludes that the downstream reading is higher than the upstream reading, which is usually indicative of a leaking valve, the confusion caused can be extensive in terms of the time and cost for replacing a valve that is not defective and is not the source of the potential problem. The actual source of the higher reading for the upstream side of the valve can originate from turbulence generated further down the piping from the valve, such as from turbulence caused by a right angle connection or even from a partial blockage of the pipe. The method of the present invention eliminates such confusion associated with checking valves by adding additional test points with which to ensure verification of the actual source of leaks or turbulence.
In accordance with the method of the invention, this is achieved by establishing a base line measurement for each valve under test. The ultrasonic sound at multiple points upstream and downstream from the valve is measured. In the preferred embodiment, the ultrasonic sound at two upstream points and two downstream points is measured. Prior to performing all measurements, a visual inspection of the valve is performed to confirm that the valve is closed or in the off position.
The measurement of the ultrasonic sound at the first upstream point is then made at a distance located X times the pipe diameter from the valve. The measurement of the ultrasonic sound at the second upstream point is made at a point located directly upstream of the valve, approximately at the pipe fitting. The two downstream measurement points are at a distance of equal relationship to the upstream test point, i.e., the first downstream measurement point is located at a distance located X times the pipe diameter from the valve, and the second downstream measurement point is located directly downstream of the valve, approximately at the pipe fitting. In the preferred embodiment, X is six (6), i.e., the first measurements are located at a point that is 6 times the diameter of the pipe away from the valve.
While touching the contact module to a measurement point to measure the ultrasonic energy at the two upstream points, the sensitivity of the dual heterodyning circuit is adjusted by turning a rotary knob on the rear of the portable device so that a liquid crystal display, also on the rear of the portable ultrasonic device, displays the dB value of the ultrasonic measurements. These initial upstream ultrasound values establish baseline measurements for the valve. The value of the ultrasonic sound at the first and second upstream points should be close to each other. In some circumstances, the second measured ultrasonic value can be slightly lower than the first measured ultrasonic value. In contemplated embodiments, the second measurement is performed without re-adjusting the sensitivity of the dual heterodyne circuit.
The level of the ultrasound at the two downstream points is then measured. The value of the ultrasound at the first downstream test point is compared to the values of the ultrasound at the upstream test points. The value of the ultrasound at the second downstream test point is measured to ensure that no ultrasonic sound is emanating downstream from the valve. If the value of the ultrasonic sound measured at the second downstream point is greater than the value of the ultrasonic sound measured at the first downstream point, then this ultrasonic sound must be xe2x80x9ctuned outxe2x80x9d or xe2x80x9cshut offxe2x80x9d to obtain a proper test of the valve. When it is not possible to tune out or shut off a downstream structure borne ultrasound, it is necessary to locate the source of the highest reading to determine its effect on the outcome of the measurements.
In accordance with the embodiments of the invention, additional ultrasound measurements further downstream are performed to assist in locating/confirming the source of the ultrasound. Hence, if the values of the ultrasound at the downstream points are higher than the values of the ultrasound at the upstream points, the valve is leaking. On the other hand, if the values of the ultrasound at the downstream points are close to or lower the values of the ultrasound at the upstream points, then the valve is not leaking, i.e., the valve is good.
By using the dual heterodyning circuit of the present invention to provide the enhanced spectrum, it becomes clear whether a detected resonance is mechanical or electrical. In addition, fault frequencies are also more easily discernible. In other words, the enhanced signal output provides a lower signal-to-noise ratio, so as to increase the ease with which frequency components are analyzed. In addition, the method of the invention eliminates the unnecessary replacements of xe2x80x9cgoodxe2x80x9d valves, which can be expensive in certain environments, such as in a nuclear plant or on a ship, where the valves are welded into place.